`MaxKey` should implement `PartialOrd<K>`

[vlopes11]

The purpose of a max key annotation is to compare to some given key to implement the data structure logic.

Since we don't have a native implementation of PartialOrd<K>, then it is always mandatory to add unrelated boilerplate code on the data structure implementations.

The implementation of a generic PartialOrd is trivial and should

• If MaxKey::Maximum(k), return k.partial_cmp(arg)
• If MaxKey::NegativeInfinity, return Ordering::Less

[ureeves]

MaxKey already implements PartialOrd<K>